The present invention relates to turbomolecular vacuum pumps.
Turbomolecular vacuum pumps are designed similar to turbines, with stator vanes and rotor blades. A significant pumping action is only obtained in the range of molecular flow (p<10−3 mbar). In the Knudsen flow range which then follows, pumping performance is reduced more and more at increasing pressure.
The pumping principle of a turbomolecular vacuum pump is based on the effect that the gas molecules which are to be pumped, obtain an impulse in the direction of the pumping action by impact with the rotor blades and stator vanes. This effect is only obtained when the circumferential velocities of the rotor blades are in the order of magnitude of the mean thermal velocity of the gas molecules to be pumped.
The mean thermal velocity of gas molecules is dependent on their molar mass. For H2 (mass 2) it amounts to approximately 1760 m/s and for nitrogen (mass 28) to approximately 470 m/s. From these figures and is apparent that the pumping properties of a turbomolecular vacuum pump are dependent on the type of gas. This not so much applies to the pumping capacity, but all the more to the compression ratio (ratio between the partial pressure of the gas component on the delivery side of the turbomolecular vacuum pump and the partial pressure of this gas component on the high vacuum side of this pump). The compression ratio of a known turbomolecular vacuum pump increases between the masses of the aforementioned gases H2 and N2from approximately 103 to 108.
The common embodiment of the blades of a turbomolecular pump is known from DE-U 72 37 362. These exhibit flat boundary surfaces. Their angle of attack (angle between the plane of the blades and a plane perpendicular to the rotational axis) increases from the suction side of the pump towards the delivery side.
From EP-A-829 645 it is known to employ rotor blades, the boundary surfaces of which are no longer flat. It is proposed to design the rear side (with respect to their direction of rotation) in a curved manner. Thus turbulences which impose a strain on the drive motor and which occur in the instance of rotor blades with flat boundary surfaces on the rear, shall be avoided.
It is the task of the present invention to improve the pumping properties of a turbomolecular vacuum pump for gases having a low specific mass.
The present application solves these problems and others.